For some applications of sintered nickel bodies, in particular as electrodes for nickel alkaline batteries, it is desirable to have the highest possible porosity consistent with adequate strength. In present commercial practice high porosity is achieved by the use of carbonyl nickel powder of the type that consists of interlocking chains of agglomerated particles. Such powder was formerly described as `Type B` powder, and is now available commercially as Inco `type 255` powder. The strength obtainable for a given porosity is generally greater the higher the temperature to which the powder is heated for sintering. However, even using Type B powder, the porosity that can be attained before the strength becomes unacceptably low does not generally exceed 80%.
To assist in achieving high porosity it has been proposed to mix the nickel powder with a spacing agent that is eliminated during or after sintering, leaving pores in its place. During the sintering process, bonds form between adjacent powder particles, and there is a tendency for the structure to collapse and so lose porosity. The spacing agent helps prevent this collapse.
According to one such proposal, in French Pat. No. 1 261 401, graphite is used as the spacing agent. A mixture of nickel powder with a large proportion, for example 50% by volume, of graphite is partially sintered in an inert atmosphere, and the resulting body is then heated in a decarburising atmosphere to remove the graphite. This process however has the disadvantage that a considerable amount of time is required to eliminate the graphite and produce the final porous structure. This greatly increases the duration of the overall process and thus increases its cost, as it is necessary to maintain the structure at an elevated temperature while the graphite is removed.
Typically the electrodes for nickel alkaline batteries are made by coating nickel powder on to a mesh support and sintering the powder under a reducing gas such as a nitrogen-hydrogen mixture, or a burnt gas which contains nitrogen, hydrogen, and small amounts, e.g. 7-8% by volume, of carbon monoxide and carbon dioxide, at a temperature in the range from 800.degree. to 1000.degree. C. The powder is conveniently applied to the support as a slurry in a liquid medium that is then removed by evaporation to produce the green body of metal powder to be sintered. To achieve the highest porosity, little or no pressure is applied before sintering.